models: remove belongs_to_creator macro.

The belongs_to_creator macro was used to initialize the creator_id field
to the CurrentUser. This made tests complicated because it meant you had
to create and set the current user every time you wanted to create an
object, when lead to the current user being set over and over again. It
also meant you had to constantly be aware of what the CurrentUser was in
many different contexts, which was often confusing. Setting creators
explicitly simplifies everything greatly.

app/models/post_flag.rb

@@ -9,7 +9,7 @@ class PostFlag < ApplicationRecord
 
   COOLDOWN_PERIOD = 3.days
 
-  belongs_to_creator :class_name => "User"
+  belongs_to :creator, class_name: "User"
   belongs_to :post
   validates_presence_of :reason
   validate :validate_creator_is_not_limited, on: :create
@@ -124,7 +124,7 @@ class PostFlag < ApplicationRecord
   def validate_creator_is_not_limited
     return if is_deletion
 
-    if CurrentUser.can_approve_posts?
+    if creator.can_approve_posts?
       # do nothing
     elsif creator.created_at > 1.week.ago
       errors[:creator] << "cannot flag within the first week of sign up"